The general objective of this proposal is to study the cytological organization of the placenta and fetal membranes and to correlate the structure with specific functional activities such as placental transport. The specific objectives are: (1) to study the absorption and subsequent localization of proteins and antibodies in the human placenta and guinea pig yolk sac using enzyme-conjugated antibodies; (2) to investigate the structure and permeability of fetal capillaries in the guinea pig and human placentas using electron dense tracer molecules; (3) to study the subcellular localization of calcium in the placenta; (4) to study the fine structure and permeability of the amnion of the guinea pig and human to electron-dense protein tracer molecules; (5) to investigate three-dimensional placental structure using scanning electron microscopy of freeze-fractured, critical point dried tissue; and (6) to investigate the comparative fine structuue of the placenta in various mammals.